The specific aims of the research described in this application are: (1) To determine the functional capacity of immature coronary collaterals (thinwalled vessels without muscular coats) immediately after coronary artery occlusion and during the first few weeks of occlusion. (2) To determine the functional capacity of mature coronary collaterals (vessels that have developed a muscular wall and have autoregulatory capacity) during the months after coronary artery occlusion. (3) To determine the time necessary for development from immature to mature forms of coronary collateral vessels after the onset of coronary occlusion. (4) To quantitate the morphologic changes occurring in coronary collateral vessels in both immature and mature forms. (5) To determine the effects of various interventions on coronary collateral flow, regional myocardial function and coronary collateral vessel morphometry. (6) To determine if there is an active growth process in the development of coronary collateral vessels using triated thymidine. These studies of the functional and structural significance of the coronary collateral circulation are designed to examine three hypotheses, i.e., (1) when coronary collaterals remodel themselves into arterioles or small muscular arteries, they becomes functionally significant; (2) there is viable functional tissue in bed at risk after coronary occlusion; and (3) the rate of growth of coronary collaterals, the mechanical function of the bed at risk and the amount of viable functional tissue in the bed at risk after coronary occlusion are augmented by exercise and drugs.